1. Field of the Invention
The invention relates to flowable coating compositions for producing scratch-resistant coatings (SRCs), based on multifunctional acrylates and methacrylates, and a method of coating molded articles, particularly articles comprised of plastics, such as organic glasses, with a clear coating in the form of a hardened thin layer which arises from polymerization of essentially polyfunctional acrylic and/or methacrylic compounds, which coating is distinguished by high scratch-resistance even under conditions of open-air weathering.
2. Description of the Related Art
According to German laid-open applications DE 21 64 716 and DE 24 55 715, multifunctional acrylate systems can be polymerized with thermally decomposable polymerization initiators, the usual oil-soluble peroxides and azo compounds; or by irradiation, e.g. by UV radiation in the presence of photoinitiators.
Preferably, UV-initiation is used, because the method can be carried out in the presence of air. According to German laid-open application DE 29 28 512, an SRC with improved adhesion to the substrate results if such a UV polymerization is carried out between 70.degree. C. and the glass transition temperature of the plastic to be coated.
Peroxide initiation is known for production of coated, scratch-resistant plates with good weather-resistance. According to European EP-B-O 245 728, the coating composition employs aliphatic peroxydicarbonates as peroxide free-radical initiators.
UV-hardenable SRCs formed from acrylic resins with added fluoroalkyl-group-containing (meth)acrylate comonomers are known from the following three Japanese patent publications:
Japanese Patent 63-196,667 (ref. CA 110:97285) discloses PC screen coatings, wherein the SRC compositions consist of the following, in a solvent mixture:
(a) 10-60% of (meth)acrylic acid esters of formula EQU H.sub.2 C.dbd.C(R)CO.sub.2 CH.sub.2 R.sub.f,
where R represents H or CH.sub.3, and R.sub.f represents a fluoroalkyl group with 1-20 C atoms; and
(b) polyfunctional acrylates;
Japanese Patent 60-151,601 (ref. CA 104:69,939) discloses coatings for plastic lenses comprised of polydiallyl compounds, wherein employed is a butylmethylketone solution with polyfunctional acrylates and dihydroperfluorohexyl acrylate in a weight ratio of 55:45; and
Japanese Patent 77-105,936 (ref. CA 88:52,095) discloses coatings for polymethacrylate lenses, wherein employed is a flowable coating resin of polyfunctional acrylic esters, methyl methacrylate, and 1 wt. % of hexafluoropropyl methacrylate based on the weight of the monomers. The fluorine-containing comonomers confer improved optical properties and improved surface smoothness on the hard coating of the polymethacrylate lens.
Coating compositions for PMMA plates are disclosed in Japanese Patent 61-258,870 (ref. CA 107:135,968), which compositions consist of diacrylates, epoxyacrylates, and fluoroalkyl-group-containing methacrylates. These compositions, however, do not allow production of hard and scratch-resistant coatings.
Plastic molded bodies which have SRCs are particularly employed in the form of plates for protecting (or protecting from) machinery, or on building exteriors, e.g. on sound-protecting walls or as glazing panels of facades, bus stop enclosure walls, advertising panels, advertising pillars, and so-called "urban furniture". In such uses transparent, low-haze plastic molded bodies are particularly used, wherewith the SRC should also be clear and transparent.
The known coating compositions according to the state of the art, employing polyfunctional acrylates, either are subject to slight surface irregularities such as uneven, rough, or structured coatings due to insufficiencies in the leveling compositions, which irregularities result in optical non-uniformities in the coating, or else such known coating compositions contain solvents, which, as is known, improve the flowability characteristics of the coating, but upon curing, due to the volatility and evaporation of the solvent, may produce undesirable surface structures and optical irregularities ("Ullmanns Encyklopaedie der Technischen Chemie" 4th Ed Vol 15 pp 595 and 676-677). Moreover, a solvent-free coating composition with polyfunctional acrylates is advantageous with regard to environmental protection and occupational health and safety.